What version of ubuntu are you on?
Ubuntu tries to limit itself to one version of each library on the system. This means only one version of mono is available on ubuntu. The version of ubuntu you have, will determine what version of .net is supported.
For example on ubuntu 12.04, only .net 4.0 applications will run, .net 2.0 applications will not run / compile by default.
You should also be aware that vb.net is not well supported in mono. c# is really the language of choice for mono. The mono runtime itself will run vb.net applications quite well but the compiler may not work. If you want to run a vb.net application on ubuntu you really need to build it on windows and then run it on Ubuntu.